parsergen: recorded a prefered shift-symbol for error recovery.
When we find there is no valid parse step, one option that we don't
currently try is to synthesize a symbol and shift it. i.e. insert a
missing symbol.
A future patch will provide a circumstance where this is the ideal
response, so here we choose a symbol which could usefully be shifted.
We choose the one that will get us closest to the end of a production.
code / ocean / commit